1. Field of the Invention
The present invention relates to the treatment of subterranean hydrocarbon-containing formations with viscous aqueous polymer solutions, and particularly, to the use of such polymer solutions for enhancing the production of hydrocarbons from subterranean hydrocarbon-containing formations penetrated by one or more well bores.
2. Prior Art
Viscous aqueous solutions containing organic polymers previously have been utilized for carrying out a variety of treatments in subterranean hydrocarbon-containing formations to increase the production of hydrocarbons therefrom. For example, in the production of oil and gas wells which also produce large volumes of salt water, the cost of producing the salt water, separating it from produced oil and gas and disposing of the salt water represents an economic loss in the operation of the wells. In order to reduce the production of salt water from such wells, viscous aqueous polymer solutions have heretofore been utilized to preferentially reduce water production from portions of the producing subterranean formations. The high molecular weight polymers or copolymers in the aqueous polymer solution plug the pores of the portions of the formation producing water and thereby substantially reduce the permeability of such portions whereby water production is also substantially reduced.
Viscous aqueous polymer solutions also have been utilized in processes for enhancing the recovery of oil from subterranean oil-containing formations. In such processes, the viscous aqueous polymer solution is injected into the formation by way of at least one injection well penetrating the formation and forced through the formation towards at least one production well penetrating the formation whereby the production of oil from the formation is increased. Such processes are usually carried out in subterranean oil-containing formations after primary recovery operations are completed, but they also can be utilized during and as a part of primary recovery operations. In the usual case, during primary recovery operations, the energy required to force oil into producing wells is supplied by the natural pressure drive existing in the formation or by mechanically lifting oil from the formation through the well bores of producing wells to the surface. At the end of primary recovery operations, a substantial quantity of oil often remains in the formation.
In enhanced recovery operations, energy for producing oil remaining in a subterranean oil-containing formation is supplied by injecting liquids or gases through one or more injection wells penetrating the formation into the formation under pressure whereby the liquids or gases drive the oil to producing wells penetrating the formation. The most common of such recovery techniques is known as water flooding wherein an aqueous liquid is injected into the formation under pressure which provides the energy and flushing action necessary to force oil in the formation to one or more production wells penetrating the formation. The efficiency of such water flooding techniques varies greatly depending upon a number of factors including variability in the permeability of the formation and the viscosity of the oil remaining in the formation. When the oil remaining in the formation is of a relatively high viscosity, aqueous flooding media of low viscosity such as fresh water or brine tend to finger through the high viscosity oil front and thereby bypass most of the available oil. In addition, typical subterranean formations contain layers of materials which often have widely varying permeability to liquid flow. Consequently, the aqueous flood medium tends to follow the course of least resistance, that is, flow through zones of high permeability in the formation from which most of the oil has been removed and thereby bypass zones of less permeability still containing substantial quantities of oil.
In order to overcome such problems, flooding media having viscosities in the order of or greater than the vicosities of oils to be displaced have been utilized. Generally, the viscosity of the water flooding medium is increased to a level at which it is close to the viscosity of the oil to be displaced, preferably equal to or greater than the viscosity of the oil to be displaced. This causes the relative mobility of the oil and the flooding medium in the formation to be comparable and the tendency of the flooding medium to finger through the oil front or bypass oil-containing portions of the formation is substantially diminished.
Viscous aqueous fluids containing organic polymers also are used as completion fluids when conducting downhole operations after initial drilling or while waiting to put the well into production. Completion fluids are used to control formation fluid pressure before the well is put on production and while mechanical work is being performed on the formation, for example, perforating, underreaming, milling or the like. The loss of completion fluids to the formation is undesirable. The fluid loss generally can be reduced by increasing the viscosity of the completion fluid.
It would be desirable to provide a method by which an aqueous fluid can be viscosified with a polymer or mixture of polymers which minimizes the quantities of polymer required to recover hydrocarbons from subterranean hydrocarbon-containing formations penetrated by one or more well bores.